The SJL/J strain of mouse which manifests a high incidence of spontaneous Hodgkin's like reticulum cell neoplasm, type-B, (RCN-B), is being studied to elucidate the possible role played by type-C RNA viruses in the pathogenesis of RCN-B in general. It was found that the virus, designated D1-MuLV, which was isolated from a mouse bearing RCN- B, showed unique type-specific envelope antigens. Injection of this virus into neonatally thymectomized BALB/c mice induced RCN-B and activation of an endogenous B-tropic type C virus. Further studies on the characteristics of these viruses and immunological reactivities of RCN-B are in progress.